1. Technical Field of the Invention
The present invention relates generally to a subband encoding and a decoding system used in data compression and decompression which are capable of working at a decreased operation load.
2. Background Art
As typical subband encoding, the MPEG1 audio is known in the art. FIG. 46 shows a conventional MPEG1 audio layer 1 encoding system. The bandwidth of digital signals 1101 sampled at a sampling frequency fs inputted to the encoding system is divided by a band splitter 101 into k subbands whose overall band is equivalent to the bandwidth of the Nyquist frequency (fs/2) of the inputted digital signals 1101. The band splitter 101 outputs k subband signals 1102 to the encoding circuit 104. Note that k is an integer. In the MPEG1 audio, the bandwidth of an input signal is divided into 32 regular subbands, but may alternatively be divided into a predetermined number of irregular subbands depending upon the design of filters.
In the MPEG1 audio layer, the subband signal in each subband is down-sampled to produce a baseband signal using a frequency modulation technique. Simultaneously, w of the input digital signals 1101 sampled at intervals of the reciprocal of the sampling frequency (1/fs) are time-frequency converted in the time-frequency converter 102 using the time-windowing in synchronism in time with the band splitter 101 to produce frequency information 1103. The length w of a time window used in the time-frequency conversion is determined as
w=(1/fr)/(1/fs)
where fr is a frequency resolution required in the frequency information 1103.
In the MPEG1 audio, the time-frequency conversion is accomplished with the Fast Fourier Transform (FFT), so that the value of w is minimum two to the nth power meeting a desired frequency resolution fr, and each time window partially overlaps with preceding and following time windows for establishing the time continuity. The frequency analyzer 103 calculates the number of allocation bits in a time period excluding the portions overlapping with the preceding and following time windows using a known psychoacoustic masking technique in a psychoacoustic mode in each of the k subbands derived by the band splitter 101 and outputs bit allocation information 1104 to the encoding circuit 104. The time period excluding the overlapping portions corresponds to a unit time length of frames.
The encoding circuit 104 determines a scale factor of each of the k subbands based on a maximum amplitude of one of the subband signals 1102 per unit frame length, normalizes the amplitude of each of the subband signals 1102 based on a corresponding one of the scale factors, re-quantizes it based on the bit allocation information 1104, produces a bit stream based on the re-quantized samples, the bit allocation information 1104, the scale factors, and frame synchronization information, and outputs the bit stream as an encoded output signal 1105.
FIG. 47 shows a conventional MPEG1 audio layer decoding system.
The signal 1106 encoded by an encoding system such as the one shown in FIG. 46 is inputted to the frame analyzer 105. The frame analyzer 105 extracts from the signal 1106 a frame, the bit allocation information, and the scale factors and provides frame analyzed information 1107 to the decoding circuit 106.
The decoding circuit 106 performs a decoding operation in each subband using the frame-analyzed information 1107 to produce subband signals 1108. The subband signals 1108 are combined in the band combining circuit 107 and outputted as a decoded output signal 1109. In order to decrease the deterioration of information with the encoding and decoding operations to reconstruct an input signal perfectly, the band combining circuit 107 needs to meet the perfect reconstruction requirements in relation to the band splitter 101 of the encoding system. To this end, a technique using QMFs is known in the art.
However, the conventional subband encoding used in the MPEG, as described in FIG. 46, performs the scale factor information producing operation, the bit allocation information producing operation, and the re-quantizing operation in each of the k subbands to construct a frame, thus encountering problems of increases in load of encoding operation and bit rate.
The subband encoding is required to perform the time-frequency conversion to analyze signals in a frequency domain for establishing the information compression based on the psychoacoustic model. The realization of high-efficiency compression without any deterioration of information requires keeping the frequency resolution completely. This also requires, when the frequency conversion is performed, performing a window function on a sample for an extended period of time.
Between the subband encoding and decoding operations, the length of a frame is determined based on the number of samples required for the windowing operation. This frame length is defined as a base unit to perform the encoding, decoding, and buffering operations. The time required for each of the operations equivalent to the frame length and the time delay by a subband split filter bank will lead to problems of increase in operation time delay with increases in sound quality and compression rate.
The subband encoding also has a problem of increase in total operation resulting from the frequency analyzing and bit allocation operations.
When the subband encoding is employed in radio transmission, the synchronization acquisition of clocks in a receiver system and synchronization of radio frame require production and detection of synchronization words. The decreasing errors occurring in a transmission path requires an additional error correcting operation, which will result in an increase in delay time in overall operations of the system resulting from buffering in each operation. The additional error correcting operation is usually performed regardless of characteristics of information produced in the subband encoding, thus resulting in an fatal error in each application even if a burst error and a bit error rate, as viewed in a long time unit, are not great.
It is therefore a principal object of the present invention to avoid the disadvantages of the prior art.
It is another object of the present invention to provide a subband encoding and a decoding system capable of decreasing a operation load and an encoding bit rate.
According to one aspect of the invention, there is provided a subband encoding apparatus which comprises: (a) a subband splitter dividing an input signal in a frequency band into subband signals; (b) a first scale factor information producing circuit measuring signal levels of the subband signals to determine scale factors and producing scale factor information indicative thereof; (c) a bit allocation information producing circuit producing bit allocation information based on the scale factor information; (d) a second scale factor information producing circuit producing scale factor flag information indicating the fact that the scale factor information has changed from that one frame earlier and updated scale factor information indicating the scale factor information which has changed from that one frame earlier; (e) a re-quantizing circuit re-quantizing the subband signals using the scale factor information and the bit allocation information to provide re-quantized output signals; (f) a frame constructing circuit constructing a frame made up of the re-quantized signal, the updated scale factor information, and the scale factor flag information and outputs the frame as an encoded output signal; and (g) a subband-limiting circuit limiting the number of subbands of the subband signals to be re-quantized by the re-quantizing circuit based on an upper limit frequency of an audible band.
According to the second aspect of the invention, there is provided a subband encoding apparatus for radio transmission which comprises: (a) a subband splitter dividing an input signal in a frequency band into a preselected number of subbands to produce subband signals; (b) a first scale factor information producing circuit measuring signal levels of the subband signals to determine scale factors and producing scale factor information indicative thereof; (c) a subband grouping circuit breaking down the subbands into a preselected number of subband groups and determining scale factors in the subband groups using the scale factor information to provide group scale factor information indicative thereof; (d) a bit allocation information producing circuit producing bit allocation information based on the group scale factor information; (e) a second scale factor information producing circuit producing group scale factor flag information indicating the fact that the group scale factor information has changed from that one frame earlier and updated group scale factor information indicating the group scale factor information which has changed from that one frame earlier; (f) a re-quantizing circuit re-quantizing the subband signals using the group scale factor information and the bit allocation information to provide re-quantized output signals; (g) a frame constructing circuit constructing a frame made up of the re-quantized signal, the updated group scale factor information, and the group scale factor flag information and outputs the frame as an encoded output signal; and (h) a subband-limiting circuit limiting the number of subbands of the subband signals to be re-quantized by the re-quantizing circuit based on an upper limit frequency of an audible band.
In the preferred mode of the invention, the subband-limiting circuit determines a minimum value of an upper limit subband number meeting a relation of ((input signal sampling frequency/2)/(the number of subbands)xc3x97(upper limit subband number))xe2x89xa7(the upper limit frequency in a given application) and determines an upper limit frequency of the subbands of the subband signals to be re-quantized by the re-quantizing circuit.
The subband splitter divides a frequency band of (sampling frequency of the input signal)/2 into 32 subbands. The subband grouping circuit breaks down 32 subbands into 6 to 20 subband groups.
The encoded output signal is outputted in the form of a frame whose length is determined by a relation of (the number of subbands)/(sampling frequency of the input signal).
The encoded output signal may alternatively be outputted in the form of a frame whose length is determined by a relation of (the number of subbands)xc3x972/(sampling frequency of the input signal).
The bit allocation information producing circuit determines a ratio of the value of the group scale factor information in each of the subband groups to the smallest value of a known minimum audible level curve in each of the subbands within the subband group. The bit allocation information producing circuit determines energy rates in all the subbands based on the ratios to obtain the bit allocation information.
The bit allocation information producing circuit may alternatively determine a ratio of the value of the group scale factor information in each of the subband groups to an average value of a known minimum audible level curve in each of the subbands within the subband group. The bit allocation information producing circuit determines the energy rates in all the subbands based on the ratios to obtain the bit allocation information.
The bit allocation information producing circuit may determine a product of the energy rate in each of the subbands and the possible number of bits to be allocated to one frame, rank all the subbands in the order of magnitude of decimals of the products, determine the remaining number of bits to be allocated, and allocate the remaining bits, in sequence, to the ranked subbands.
The bit allocation information producing circuit may produce the bit allocation information using weighting coefficients in a frequency domain.
The bit allocation information producing circuit may alternatively produce the bit allocation information using weighting coefficients each provided for the scale factor information in one of the subbands.
The frame constructing circuit may set a length of the frame to that of a radio transmission frame and pads a synchronization word required for radio transmission in the frame.
The frame constructing circuit transmits frames each made up only of synchronization words for synchronization acquisition at regular intervals.
The frame constructing circuit may alternatively transmit frames each made up only of the group scale factor information at regular intervals.
The frame constructing circuit may alternatively transmit frames each made up of the group scale factor information and synchronization words for synchronization acquisition at regular intervals.
An error correction encoding circuit may further be provided which performs an error correction encoding operation on the encoded output signal.
The error correction encoding circuit performs error correction encoding operations having different error correcting capabilities on data in a frame of the encoded output signal according to error resistances of the data.
The error correction encoding circuit may use a BCH code.
The error correction encoding circuit may alternatively use a convolutional code.
The error correction encoding circuit may use different error correcting codes according to the error resistances of the data.
The error correction encoding circuit may use both a BCH code and a convolutional code.
The error correction encoding circuit may provide a bit in a frame of the encoded output signal which undergoes no error correction encoding operation according to weights of data contained in the frame.
The error correction encoding circuit may perform the error correction encoding operation on the encoded output signal regardless of a bit length of the updated scale factor information which changes every frame.
A rearranging circuit may further be provided which rearranges output signals produced by re-quantizing the subband signals for minimizing adverse effects of code errors on a decoding operation.
The frame constructing circuit may perform an interleaving operation on the encoded output signal.
According to the third aspect of the invention, there is provided a subband decoding apparatus which comprises: (a) a frame analyzer establishing synchronization of frames of an inputted subband encoded signal to extract therefrom re-quantized signals, scale factor flag information indicating the fact that scale factor information has changed from that one frame earlier in a subband encoding operation of the inputted subband encoded signal, and updated scale factor information indicating the scale factor information which has changed from that one frame earlier in the subband encoding operation; (b) a scale factor information producing circuit producing the scale factor information in all subbands of the inputted subband encoded signal using the updated scale factor information and the scale factor flag information; (c) a bit allocation information producing circuit producing bit allocation information based on the scale factor information produced by the scale factor information producing circuit; (d) a subband signal producing circuit receiving the re-quantized signals to produce subband signals using the scale factor information and the bit allocation information; and (e) a band combining circuit combining the subband signals to produce a decoded output signal.
According to the fourth aspect of the invention, there is provided a radio transmission subband decoding apparatus which comprises: (a) a frame analyzer establishing synchronization of frames of an inputted subband encoded signal to extract therefrom re-quantized signals, group scale factor flag information indicating the fact that group scale factor information on scale factors in subband groups into which subbands are broken down in a subband encoding operation of the inputted suband encoded signal has changed from that one frame earlier, and updated group scale factor information indicating the group scale factor information which has changed from that one frame earlier in the subband encoding operation; (b) a group scale factor information producing circuit producing the group scale factor information in the subband groups of the inputted subband encoded signal using the updated scale factor information and the scale factor flag information; (c) a bit allocation information producing circuit producing bit allocation information based on the group scale factor information produced by the group scale factor information producing circuit; (d) a subband signal producing circuit receiving the re-quantized signals to produce subband signals using the group scale factor information and the bit allocation information; and (e) a band combining circuit combining the subband signals to produce a decoded output signal.
In the preferred mode of the invention, the system may further comprises a muting circuit which mutes frames contained at regular intervals in the inputted subband encoded signal when each of the frames is made up of either or both of the scale factor information and synchronization words for synchronization acquisition and a data interpolation circuit which performs data interpolation of digital signals undergoing a decoding operation.
The radio transmission subband decoding system may alternatively comprise a muting circuit which mutes frames contained at regular intervals in the inputted subband encoded signal when each of the frames is made up of either or both of the group scale factor information and synchronization words for synchronization acquisition and a data interpolation circuit which performs data interpolation of digital signals undergoing a decoding operation.
The subband decoding system may alternatively comprise a muting circuit which mutes frames contained at regular intervals in the inputted subband encoded signal when each of the frames is made up of either or both of the scale factor information and synchronization words for synchronization acquisition and a data interpolation circuit which performs data interpolation of analog signals undergoing a decoding operation.
The radio transmission subband decoding system may further comprise a muting circuit which mutes frames contained at regular intervals in the inputted subband encoded signal when each of the frames is made up of either or both of the group scale factor information and synchronization words for synchronization acquisition and a data interpolation circuit which performs data interpolation of analog signals undergoing a decoding operation.
The subband decoding system may comprise a muting circuit which mutes frames contained at regular intervals in the inputted subband encoded signal as a function of the number of bits in each of the frames and a data interpolation circuit which performs data interpolation of digital signals undergoing a decoding operation.
The radio transmission subband decoding system may comprise a muting circuit which mutes frames contained at regular intervals in the inputted subband encoded signal as a function of the number of bits in each of the frames and a data interpolation circuit which performs data interpolation of digital signals undergoing a decoding operation.
The suband decoding system may comprise a muting circuit which mutes frames contained at regular intervals in the inputted subband encoded signal as a function of the number of bits in each of the frames and a data interpolation circuit which performs data interpolation of analog signals undergoing a decoding operation. The radio transmission subband decoding system may comprise a muting circuit which mutes frames contained at regular intervals in the inputted subband encoded signal as a function of the number of bits in each of the frames and a data interpolation circuit which performs data interpolation of analog signals undergoing a decoding operation.
The decoded output signal may be subjected to a deinterleaving operation during analysis of a frame of the input signal.